Obsessive-compulsive disorder (OCD) is a common psychiatric disorder with a lifetime prevalence of 2.5 percent. Family studies indicate that OCD and sub-clinical obsessive-compulsive symptoms are familial. About one third of cases have onset by age 15 years. Recent family studies of OCS suggest that an early onset is associated with increased familial risk. The long-term goal of this project is to improve our understanding of the etiology of OCD by identifying susceptibility loci involved in early-onset OCD and determining the expression of these loci. An initial collection of families with early-onset OCD will be expanded through ascertainment of an additional 120 families with early-onset probands in which there is an affected sibling or second-degree relative. DNA from family members will be genotyped with mapped microsatellite markers and amplified by multiplex PCR spaced about every 10 cM. Parametric and nonparametric linkage analyses will be performed. Once evidence for linkage between early-onset OCD and a genetic marker is found, more precise gene localization will be pursued.